The present invention relates to a pilot valve, for example of the type which is commonly used to control a pressure reducing valve in water and gas supply systems.
FIG. 1 illustrates the use of a “single chamber” pilot valve 1 to control a pressure reducing valve (PRV—shown schematically as item 2) as commonly used in a water supply system. In the context of a gas supply system such a pressure reducing valve is normally known as a “regulator” or “governor”, but herein the single term “PRV” is used for simplicity as referring to both types of system. The fluid to be controlled (usually water or gas, and in this example will be taken to be water) flows along the main pipe 3 through the PRV. The outlet pressure (Po) is usually less than the inlet pressure (Pi) due to the action of the PRV.
The amount of pressure reduction is controlled by operation of the PRV under control of pilot valve 1. An auxiliary flow pipe 4 carries water from the inlet of the PRV to the control chamber 5 of the pilot valve 1 and then back to the outlet of the PRV. Prior to entering the control chamber 5, the water passes through a venturi chamber (or primary orifice) 6 or, more correctly in the context of a gas supply system, an inspirator 6 and the water pressure (Pv) at the outlet side 7 of the chamber or inspirator controls the PRV.
The flow of water through the control chamber 5 is controlled by a gate mechanism 8 which is linked to a diaphragm 9. A spring 10 applies force to the rear of the diaphragm 9 and the amount of force supplied by the spring may be varied by an adjustment screw 11.
In a steady state situation (where Po remains constant) the water pressure in the control chamber 5 will be balanced by the force generated the spring and the gate 8 will remain in a constant position. Thus the flow through the auxiliary pipe 4 will remain constant and PV will remain constant.
If the control pressure (Po) falls, the spring 10 causes the gate 8 to open further and the flow through the auxiliary pipe increases. Accordingly, the flow through the venturi 6 also increases which results in pressure Pv decreasing, causing the PRV to open further. This results in the control pressure Po rising again and the system should then reach a steady state again at the previously set value of Po.
In order to provide an improved control system, the present applicant has already disclosed a system which uses a “dual chamber” pilot valve in European Patent No. 574241. FIG. 2 shows an example of a system utilising a “sandwich plate” dual chamber pilot 20. The pilot valve 20 performs the same general function in the control system as the pilot valve of FIG. 1 but in this example the adjustment previously provided by adjustment screw 11 is effectively supplemented by an adjustment using a control pressure (Pc). As further relevant background art may be mentioned the gas supply pressure control apparatus as disclosed by the present applicant in GB-A-2252848.
The pilot valve 20 includes a second chamber 21 which is effectively divided into two portions 22 and 23 by a wall 24. The control pressure Pc effectively acts against the force of spring 10 by virtue of diaphragm 26. As with FIG. 1, the spring is mechanically connected by arm 28 to a gate mechanism 8 which performs the same function as previously. The arm 28 passes through wall 24 and the aperture through which it passes is sealed by a seal 29 so that chamber 23 does not contain any water but instead is vented to the atmosphere.
If the control pressure Pc remains constant, then the system operates as explained with reference to FIG. 1. However, if the control pressure Pc is reduced then the gate 8 will open further thereby reducing pressure Pv and increasing the outlet pressure Po. This is usually referred to as a “failsafe” system since in the event that the control pressure fails i.e. falls to zero, the outlet pressure Po will be set to its maximum value.
FIG. 3 illustrates an alternative but mechanically equivalent “dual chamber” pilot valve arrangement to that shown in FIG. 2. The arrangement of FIG. 3 is sometimes referred to as a “pancake adapted” pilot. In this arrangement, the second chamber 31 is located at the base of the pilot 30. As with the arrangement of FIG. 2, the second chamber 31 is divided by a diaphragm 34 into two chambers 32 and 33 and the control pressure Pc is applied to chamber 32. The diaphragm 34 is mechanically linked via an arm 35 to the gate mechanism 8 but is not rigidly limited to the gate or spring. The arm 35 presses into control chamber 5 via an aperture which is again sealed with seal 36.
In the embodiment of FIG. 3, the control pressure Pc again opposes the force produced by the spring 10 and so the control system effectively operates in an identical manner. In other words, if control pressure Pc is reduced then the outlet pressure Po is increased.
One advantage over the FIG. 3 arrangement as opposed to the FIG. 2 arrangement is that the additional chamber 31 can effectively be retrofitted to a single chamber pilot valve. However one disadvantage with the dual chamber pilot valves of FIGS. 2 and 3 is that in both cases a seal needs to be provided in order that the control fluid is prevented from entering the second part of the additional chamber i.e. that part of the chamber to which the control pressure is not applied. The provision of such a seal can be difficult and deterioration or failure of the seal may lead to reduction in performance of the pilot valve or leakage therefrom. Furthermore, the friction caused by the seal can in turn create a frictional error in the quality of the pilot valve control.
FIG. 4 shows a further “hydraulic” dual chamber pilot valve arrangement. As with the previous embodiments, a second chamber 40 is provided which is divided by a diaphragm 41 into two parts 42 and 43. The control pressure Pc is applied to part 42 of the second chamber 40 and part 43 is connected to the spring chamber which is vented to the atmosphere. As before, the diaphragm 41 is mechanically connected to the gate 8, in this case via the spring 10.
However, unlike the embodiments of FIG. 2 and FIG. 3 in the embodiment of FIG. 4 the control pressure Pc acts in the same direction as the force of the spring 10, rather than against it. This means that the control system works in the opposite way to that of FIGS. 2 and 3 i.e. if the control pressure Pc is reduced then the gate 8 closes further, the venturi pressure Pv increases causing the PRV to close further and the outlet pressure to drop. This arrangement is not considered to be “failsafe” since a loss of control pressure Pc would result in the lowest possible outlet pressure Po. This is sometimes referred to as a “direct acting” control system rather than the “reverse acting” control systems of FIGS. 2 and 3.
The present invention aims to provide a pilot valve of the “reverse acting” type but which eliminates the need for a seal.